1. Field
Embodiments of the invention are directed generally to a distributed antenna system (DAS) and method for supporting multi-sector and/or multiple-input and multiple-output (MIMO) systems. More specifically, embodiments of the invention are directed to a DAS for supporting multi-sector and/or MIMO systems with a reduced number of optical fibers utilizing wavelength division multiplexing (WDM), and in particular dense wavelength division multiplexing (DWDM), and a method of implementing such a DAS.
2. Description of Related Art
Distributed antenna systems (DAS) are generally used to expand wireless coverage in settings such as buildings, campuses, stadiums, hospitals, tunnels, and the like, where there may be shadow regions or areas with reduced signal strength. A DAS generally includes a number of head-end units (HUs) that interface with one or more base stations (BTS) and a plurality of remote units (RUs) that are distributed throughout a given service area to provide coverage in the service area. The DAS is used to amplify the signal strength to improve coverage, particularly in the shadow regions or areas with reduced signal strength.
FIG. 1 illustrates a block diagram of a general analog optical DAS. The optical DAS in FIG. 1 includes a base station (BTS) 101, a head-end unit (HU) 300, and a plurality of remote units (RUs) 400, the latter of which are distributed in different regions of a specified service area. The HU 300 interfaces with the BTS 101 and the RUs 400 through fiber optic lines or cables connected to the HU 300.
The head-end unit 300 includes a head-end RF unit (HRFU) 301, an electrical-to-optical (E/O) converter 302, an optical-to-electrical (O/E) converter 307, a wavelength division multiplexer (WDM) 305, and an optical splitter 306.
The HRFU 301 adjusts the level of a downlink signal 201 received from the BTS 101 to a suitable level and transfers the signal to a number of distributed RUs 400. The HRFU 301 also adjusts the level of an uplink signal received from the OLE converter 307 to a suitable level and transfers the signal 202 back to the BTS 101.
The E/O converter 302 transforms or converts a downlink RF signal into a downlink optical signal 303, where the downlink optical signal 303 may have a different wavelength for each E/O converter 302.
The O/E converter 307 transforms or converts an uplink optical signal 304 into an uplink RF signal. Different O/E converters 307 may utilize optical signals 304 with different wavelengths. In this manner, the HU 300 may both convert a downlink RF signal into a downlink optical signal, and may also convert an uplink optical signal to an uplink RF signal.
The wavelength division multiplexer (WDM) 305 combines a plurality of optical signals with different wavelengths received from one or more E/O converters into a combined optical signal with multiple wavelengths. The WDM 305 also splits combined optical signals with multiple wavelengths received from a single optical cable into a plurality of separate optical signals transmitted through different optical paths based on the different wavelengths.
The optical splitter 306 splits an optical signal with multiple wavelengths from an optical cable connected to the WDM 305 into a plurality of optical signals with multiple wavelengths, transmits the split or divided optical signals with multiple wavelengths through a plurality of optical cables to different remote units 400. The optical splitter 306 also combines optical signals with multiple wavelengths from the RUs 400 through a plurality of optical cables into a combined optical signal with multiple wavelengths and transmits the combined optical signal through a single optical cable to the WDM 305.
Each of the remote units 400 includes a wavelength density multiplexer (WDM) 401, an optical-to-electrical (O/E) converter 404, an electrical-to-optical (E/O) converter 405, and a remote RF unit (RRFU) 406.
The wavelength division multiplexer (WDM) 401 splits a combined optical signal with multiple wavelengths received from a single optical cable into a plurality of separate optical signals transmitted through different optical paths based on the different wavelengths. The WDM 401 also combines a plurality of optical signals with different wavelengths received from the E/O converter 405 into a combined optical signal with multiple wavelengths.
The O/E converter 404 transforms or converts a downlink optical signal 402 into a downlink RF signal. The downlink optical signal 402 may have a different wavelength for each O/E converter 404. Meanwhile, the E/O converter 405 transforms an uplink RF signal into an uplink optical signal 403, where each uplink optical signal may have a different wavelength based on the E/O converter 405.
The remote RF unit (RRFU) 406 adjusts the level of the downlink signal received from the O/E converter 404 to provide proper coverage for a particular area, and monitors the level of uplink signals received from multiple mobile stations (MSs) in the designated service area, and adjusts the signals to optimize the signal levels for the MSs. The RRFU 406 also filters out-band spurious signals, for example, via a duplexer filter.
Meanwhile, in some settings such as campuses and stadiums, traffic loads for voice or data calls may vary or fluctuate more greatly during peak usage times based on the time of day. Due to the wide service areas of such settings, and various other factors such as multiple building structures and the number of users in the service area or other predefined area, sometimes multiple sectorization of such settings, where the service area is divided into multiple sectors, is desirable to support sufficient wireless coverage and throughputs. Typically, each such sector is connected to separate base stations with separate capacities.
FIG. 2 is a block diagram illustrating a general distributed antenna system (DAS) which supports multiple sectors. As seen in FIG. 2, each head unit (HU) 300, 310, 320 is respectively connected to a separate sectorized base station (BTS) 101, 102, 103, as well as its own plurality of sectorized remote units (RUs) 400, 410, 420. Each separate sector is arranged similarly to the DAS illustrated in FIG. 1, and the descriptions of similar parts will therefore be omitted.
As can be seen in FIG. 2, as there is no cross-communication between the separate sectors of FIG. 2, the infrastructure does not support adjusting the traffic loads between the sectors, and therefore, when the traffic at one sector exceeds a bandwidth allotment or capacity within that sector, voice calls and/or data communication in that sector suffer. If the traffic loads in a particular section exceed the capacity supported by an associated sectorized BTS, the RUs connected to that particular BTS will not be able to support the excess traffic unless the capacity of that sectorized BTS is increased. Meanwhile, capacity in the other sectors may be lower, and bandwidth at the other sectorized BTSs may remain unused and underutilized.
FIG. 3 illustrates a block diagram of a general optical distributed antenna system (DAS) which is configured to support multiple-input and multiple-output (MIMO) systems. The head-end units (HUs) and the remote units (RUs) are configured similarly to those in FIGS. 1 and 2, and therefore, the descriptions of similar parts will be omitted.
The DAS in FIG. 3 allows for sharing of bandwidth or capacity between the different BTSs 101, 102. However, in these systems, for the DAS to share bandwidth between the BTSs, the DAS requires separate optical cables to be routed between each HU and each RU. For example, in FIG. 3, a 2×2 MIMO DAS which includes two HUs 300, 310, and two remote RF units (RRFUs) 406, 415 per RU 400, requires two optical cables to be separately routed from each of the two HUs 300, 310 to the separate RRFUs 406, 415, respectively, in each RU 400. In this manner, a general DAS provides two separate and distinct RF paths to support a 2×2 MIMO architecture. Therefore, in a general DAS supporting 2×2 MIMO, two optical cables must be routed to each RU 400, while in a 4×4 MIMO DAS, four optical cables must be routed to each RU 400. As such, with larger systems, such an arrangement would become prohibitive. For example, larger systems will require increased expenses for multiple optical cable installation and maintenance.